/Engine/Source/Editor/UnrealEd/Public/Tools/LegacyEdModeWidgetHelpers.h
Specifies a function used for validation of the current value of a property. The function returns a string that is empty if the value is valid, or contains an error description if the value is invalid
static const [FName](API\Runtime\Core\UObject\FName) MD_ValidateWidgetUsing;